This invention relates to light engines for color projection display systems, and more particularly relates to a scrolling color light engine for use in a single panel color projection display system.
A single panel scrolling color projection display system is characterized by a single light modulator panel such as a liquid crystal display (LCD) panel having a raster of individual picture elements or pixels, which panel is illuminated by horizontally elongated red, green and blue illumination bars or stripes. The stripes are continuously scrolled vertically across the panel while the rows of pixels are synchronously addressed with display information corresponding to the color of the then incident stripe. The modulated scrolling red, green and blue stripes are then projected onto a display screen to produce a visually integrated full color display. See, for example, U.S. Pat. No. 5,410,370, xe2x80x9cSingle panel color projection video display improved scanningxe2x80x9d issued to P. Janssen on Mar. 25, 1994, and U.S. Pat. No. 5,416,514, xe2x80x9cSingle panel color projection video display having control circuitry for synchronizing the color illumination system with reading/writing of the light valvexe2x80x9d issued to P. Janssen et al. on May 16, 1995.
Such single panel systems are to be distinguished from the more conventional three-panel systems, in which separate red, green and blue beams each fully illuminate and are modulated by a separate panel. The modulated beams are then superimposed on a display screen to produce a full color display. See, for example, U.S. Pat. No. 5,917,561, xe2x80x9cLiquid-crystal image projecting apparatus having a color purity correction filterxe2x80x9d issued to Hatanaka on Jun. 29, 1999.
Light engines for both single-panel and three-panel color projection display systems commonly utilize high intensity arc lamps to provide the level of intensity needed for a bright display, as well as dichroic filters to split the lamp light into red, green and blue components for modulation, and then to recombine the components for display.
A problem with both systems is a color varation across the image caused by the angle sensitivity of the cut-off wavelengths of the dichroic filters. That is, the wavelength at which light is reflected from the surface of the dichroic filter changes with the incident angle of the reflected light. This problem is accentuated in systems employing certain high efficiency arc lamps having emission peaks in their spectral output, such as ultra-high-pressure (UHP) lamps having a strong orange component, and xenon lamps having significant energy in both the orange and blue spectral regions.
This problem is addressed for a three-panel system in U.S. Pat. No. 5,917,561 by inserting a correction filter into the optical path at an angle of 10 degrees, which is said to compensate for the angle shift on the dichroic filters by generating a reverse effect of angle shift (column 5, lines 33 et seq.). However, in a single-panel system, the angle shift is exacerbated by the so-called xe2x80x9cbeamsteering effectxe2x80x9d, illustrated schematically in FIGS. 1A through 1C.
In these figures, a single color beam is illustrated scrolling across the panel, although in practice, each color beam red, green and blue is scrolled through a separate optical path. The ray bundle 2 that forms a stripe-shaped beam 1 is refracted by a glass prism 6, rotating about an axis normal to the plane of the drawing, and thence directed by field lenses 8, 10, and 12 through recombining dichroic filters C and D and polarizing prism 14 to panel 16.
The rotating prisms cause the stripes to scroll across panel 16 from top to bottom, causing a beamsteering effect in which the incident angles of the stripes on filters C and D to vary over a range of values, the extremes of which are larger than would be encountered in a three-panel system, in which the beam positions are fixed. For example, in the illumination path shown in FIG. 2A, the beamsteering effect results in incident angle variations at filters C and D of xc2x18.1 and xc2x15.3 degrees, respectively. For a typical angle sensitivity of xe2x88x921.4 nm/degree, such variations translate in cut-off wavelength variations of up to 23 nm, which can have a significant effect upon the transmitted spectrum.
One solution to this beamsteering problem is provided in U.S. Pat. No. 5,999,321, xe2x80x9cDichroic filters with low nm per degree sensitivityxe2x80x9d, issued to Bradley on Dec. 7, 1999. However, the thin film dielectric stack prescribed for these filters include layers as thin as xcx9c{fraction (1/16)}th of a wave and must be fabricated with a high degree of accuracy.
Accordingly, it is an object of the invention to provide a solution to the beamsteering problem which does not require the use of dichroic filters having a low angle sensitivity.
In accordance with one aspect of the invention, a light engine for a scrolling color projection system includes at least one source filter to reject unwanted components from the spectral output of the light source prior to encountering any beamsteering effects, and beamsplitting filters having filter characteristics chosen to mask the beamsteering effect on the recombining dichroic filters.
In general, the cut-off wavelengths of the splitting filters are chosen to cut off the lower edge of the red spectrum and the upper edge of the blue spectrum beyond the corresponding filter edges of the recombining filters, so that the transmitted red and blue spectral components are relatively unaffected by the varying filter edges of the recombining filters. This results a relatively broad green spectrum, since the green spectrum is defined by the remaining wavelengths between red and blue. The upper and lower wavelength edges of the green spectral component can then be clamped by using one or more additional xe2x80x9csourcexe2x80x9d filters to reject light from the illumination between the red and blue wavelength regions.
In its broadest aspects, the light engine comprises:
a light source for providing an output beam;
first and second splitting filters for splitting the output beam into red, green and blue components;
field lenses to form the red, green and blue components into beams;
first, second and third rotating prisms for scrolling the red, green and blue beams;
first and second recombining filters for recombining the red, green and blue beams; and
at least one filter for eliminating undesired spectral components from the output beam prior to scrolling;
the splitting filters having filter edges chosen to mask beamsteering effects on the recombining filters.
In one embodiment,
the first splitting filter reflects red light and transmits green and blue light from the source;
the second splitting filter reflects green light and transmits blue light;
the first recombining filter reflects green light and transmits red light;
the second recombining filter reflects red and green light and transmits blue light.
In the above embodiment, if a UHP lamp having a significant orange component is chosen as the light source, the preferred characteristics of the filters are as follows:
the source filter has a cut-off is about 568 nm;
the first splitting filter has a cut-off range from about 593 to 604 nm; and
the second splitting filter has a cut-off of about 501 nm;
the first recombining filter has a cut-on of from about 574 to 579 nm; and
the second recombining filter has a cut-off of about 518 nm.
In the above embodiment, filter O is preferable placed in the green/blue light path, between filters A and B, allowing removal of the orange peak using a simple low pass filter. Alternatively, filter O could also be positioned in the red section of the light path, assuming that filter A is designed to redirect both the red and orange spectral regions. Dichroic notch filters could also be included in the light path to block the orange peak although notch filters generally have a higher insertion loss than standard low-pass or high-pass filters.
Such a light engine for a single-panel scrolling-color projection system including a source filter for filtering out unwanted components from the output spectrum of the source lamp prior to scrolling, and beam splitting filters having filtering characteristics chosen to mask the beamsteering effects caused by the angle sensitivity of the recombination filters, produces color stripes which maintain stable colors during scrolling, without the use of splitting and recombining filters with low angle sensitivities.
In accordance with another aspect of the invention, a scrolling color projection system incorporates a light modulator panel; a light engine as provided in claim 1 for illuminating the panel; drive means for driving the light modulator panel in accordance with a display signal; and a projection lens for projecting the modulated illumination onto a display surface. Such a display system incorporating the light engine of the invention has improved color uniformity across the panel.